The 5,6-dihydroxyindoles have been found to be very useful in dyeing hair, and particularly human hair on the head. Moreover, because of their inherent instability they have presented problems in their practical application. These instability problems are attested to in U.K. patent 797,174, U.S. Pat. No. 3,194,734 and U.S. Pat. No. 4,208,183 which are concerned with ways and means for preparing stable compositions containing these indole dyes.
U.K. patent 797,174 describes a process for preparing a dye composition containing 5,6-dihydroxyindoles and dyeing hair therewith. In accordance with this patent the 5,6-dihydroxyindoles are employed in the form of the 5,6-diacetoxyindole compounds because of the stability of the latter. The diacetoxy compound is then deacetylated in aqueous solution with an alkalizing agent like monoethanolamine immediately prior to use, and the composition so formed is applied to hair. It is to be noted that this process is carried out in the presence of atmospheric oxygen.
U.S. Pat. No. 3,194,734 to Seemuller describes a process for dyeing hair with methyl substituted dihydroxyindoles which provide lighter color dyeouts than are provided by dihydroxyindole itself. The patent further teaches that alkaline solutions suitable for hair dyeing can be prepared immediately prior to dyeing by operating in a nitrogen atmosphere or by adding antioxidants such as thioglycollate, sulfite or the like. Based on the teachings of the prior art such solutions are not believed to have storage stability. The examples of the patent clearly show that, in presence of antioxidants, only weak colors such as light ash blond, gray, and bluish-gray are obtained by use of 5,6 dihydroxyindole unless extremely high levels of dye are employed. With the process of the present invention dark black dyeouts are produced, even after prolonged storage using very low levels of dye.
U.S. Pat. No. 4,208,183 teaches the preparation of storage stable solutions of dihydroxyindoles in alcoholic solvents. Prolonged storage of dihydroxyindoles is possible by this method, but these alcohol solutions are not suitable for dyeing hair, which is a serious disadvantage. Therefore, these storage stable solutions of the dihydroxyindoles must be mixed with a suitable cosmetic vehicle immediately prior to dyeing. The preferred method disclosed involves a dual compartment aerosol package. The reference does not disclose employing or storing the dihydroxyindoles under anaerobic conditions.
The present invention allows for storage in an aqueous medium which is suitable for dyeing hair directly rather than requiring a complex change of solvent and delivery system.
While 5,6-diacetoxyindole can be used as a stable precursor for 5,6-dihydroxyindole, its use still presents difficulties since the 5,6-dihydroxyindole is sensitive to oxidation by atmospheric oxygen. This can lead to high-loss of the dihydroxyindole due to its auto-oxidation outside the hair.